Hydraulic apparatus



' Sept. 19, 1967 R. WALTERS HYDRAULIC APPARATUS Filed July 20, 1965 v Fig. 1.

INVENTOR Roma 1. 0 14 4 L TEES H ATTORNEY United States Patent 3,342,213 HYDRAULIC APPARATUS Ronald Walters, Wembley, England, assignor to The S erry Gyroscope Company Limited, Brentford, Middlesex, England, a company of Great Britain Filed July 20, 1965, Ser. No. 473,413 Claims priority, application Great Britain, July 22, 1964, 29,569/ 64 3 Claims. (Cl. 137-625.6)

The invention relates to hydraulic servo systems and in particular to spool valves for use in such systems.

In a hydraulic servo system the flow of hydraulic fluid to an actuator is generally controlled by a spool valve in which the spool has raised lands co-operating with ports drilled in the walls of the valve body, or in a sleeve forming a lining for the valve body. The arrangement of the lands and ports is such that in the neutral position of the valve the ports are closed by the lands, and a small displacement of the valve spool in one or the other sense allows a flow of hydraulic fluid in one or the other direction through the actuator.

The ports are normally circular in section, and in order to obtain a linear relation between the spool displacement and hydraulic flow the lands are sometimes made to underlap the valve ports, that is to say they are slightly narrower in width than the diameter of the ports. This results in the valve having a slight hydraulic leakage in its neutral position, but usually this is of no importance.

The hydraulic gain of the valve, that is to say the flow per unit displacement of the spool, is inversely related to the gain of the other parts of the servo loop, which are usually electrical, since the overall gain is a design factor determined by requirements of stability and dynamic response of the system.

The electrical gain in an electro-hydraulic servo system usually controls the steady-state errors of the system and therefore, for accuracy of position, for example, in a hydraulic positioning system, the electrical gain should be as high as possible, the hydraulic gain being correspondingly low. 0n the other hand the rapidity of response to larger error signals is determined by the flow through the valve, and for these larger signals it is an advantage for the hydraulic gain to be high and the electrical gain correspondingly low. In principle it is possible to control the gain of the valve at various displacements by suitably shaping the ports, but in practice it is diflicult to manufacture non-circular ports with the accuracy required. An object of the present invention is to provide a form of hydraulic valve of convenient construction and non-linear response, and a servo system incorporating such a valve.

According to the present invention in a spool valve each set of metering ports for the hydraulic fluid comprises at least one port of circular section and of a diameter slightly exceeding the width of the spool lands, and at least one port of circular section and of a diameter less than that of the spool lands. Preferably there are two or more ports of each size, and they are symmetrically disposed about the spool axis, to avoid unbalanced radial forces.

In operation, for very small displacements the larger, underlapped ports only are effective, and give a linear response in the usual manner. As the displacement increases the ports of smaller size begin to be uncovered and increase the hydraulic gain of the valve.

The invention is applicable in particular to electrohydraulic valves in which the valve spool is driven by a torque motor either directly or through a pilot stage and the electrical drive circuits may include an amplifier having a non-linear response such as to compensate for the characteristic of the valve and maintain a substantially constant open loop gain for the system as a whole. Usually 3,342,213 Patented Sept. 19, 1967 two sizes of valve port will be suflicient for most applications, but the number of ports of each size may be varied according to the particular characteristic required.

The invention will be further described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a sectional view of a hydraulic portion of an electro-hydraulic servo valve according to the invention, and

FIGURE 2 is a schematic development of one set of ports responsible for the metering of the flow of output fluid.

Referring first to FIG. 1, the valve consists of a valve body 1 within which is shrink-fitted a central sleeve 2 in which the valve ports of the main valve are drilled and having passages for the supply to them of hydraulic fluid. Within the sleeve 2 is a spool 3 of the main valve, having a pair of raised lands 4 and 5 which respectively cooperate with sets of ports 6 and 7 formed in the sleeve. Hydraulic fluid under pressure is supplied through a passageway 8 to the space between the lands of the spool and is exhausted to a tank at low pressure from spaces 9 and 10 outside the lands, the lands 4 and 5 co-operating with the sets of ports 6 and 7 meter the flow of fluid to a hydraulic jack or motor, displacement of the valve spool allowing fluid under pressure to pass into one set of ports, and fluid returned from the jack or motor to pass to the tank from the other. As so far described, this part of the valve is conventional in structure.

According to the invention the ports 6 and 7 are arranged as shown in the development of FIG. 2. In this figure the ports consist of a pair of orifices 14, 15 whose diameter exceeds the width of the face of the valve land 16 by a few ten thousandths to a few thousandths of an inch according to the size of the valve. In between these larger ports 14, 15 are two series each of three smaller orifices 17, 18, 19 of uniform size and having their axes all in the same plane, but all of somewhat smaller diameter than the width of the spool land 16 The valve shown is of a two-stage type in which the spool is bored at one end to receive a sleeve 11 Within which works a smaller spool 12 forming a second pilot valve arranged within the main spool. The other end of the main valve spool is bored to receive a compensating piston 13 the inner end of which is exposed to the main hydraulic pressure. The outer end of this piston bears against an abutment (not shown in the drawing) when the valve is assembled in place.

The space between the spool 3 and the main sleeve 2 is closed by a flanged ring 20 having ports for hydraulic fluid, and the spool 3 also has a further raised land 21, the space between the land 21 and the end of the ring 20 forming a further hydraulic cylinder to which fluid can be admitted through ports under the control of the pilot valve spool 12. The area of this annular cylinder is arranged to be twice that of the cross section of the compensating piston 13.

A torque motor (not shown) is used to move the pilot valve spool 12 in response to an electrical input signal. When the pilot valve spool 12 is in its neutral position, the pressure in the annular space between the ring 20 and the land 21 is half that of the hydraulic supply and is just counterbalanced by the supply pressure acting on the compensating piston 13, which is of half the crosssectional area.

In operation the application of an electrical signal to the torque motor displaces the spool 12 of the pilot valve, causing the hydraulic pressure in the annular space to be raised or lowered, as the case may be, and consequently causing the main valve spool 3 to be displaced to follow up movement of the pilot valve spool 12.

For sutficiently small displacement of the main valve spool, the small orifices 17, 18, 19 of FIGURE 2 are not uncovered, and the larger orifices 14 and 15 are operating in the normal manner with a slight underlap giving a linear variation of flow rate. If the displacement increases further, the edges of the smaller orifices 17, 18, 19 become uncovered by the lands 4 and 5, giving a further increase in flow rate.

In one example of a valve according to the invention designed for use at a pressure of 3000 lbs. to the square inch and a maximum flow rate of 40 gallons per minute, the bore of the valve body was 1.5 inches, the width of the lands 0.100, and the valve was provided with two rings of ports each consisting of four holes 0.104" diameter and twelve holes 0.094" diameter with their axes in the central planes of the lands in the valve neutral position. For this valve the ratio of the gain at maximum valve travel to that at valve neutral was 6:6.

In another example, designed to have a higher gain ratio and a maximum flow rate of 60 gallons per minute, the dimensions were similar, but each ring of ports consisted of two holes of the larger size and twenty-two of the smaller. This valve had a gain ratio of 19:6.

The servo valve is arranged to 'be driven by an electrical circuit incorporating either a non-linear amplifier or a non-linear correction means which compensates for the change in hydraulic gain of the valve with displacement and so maintains the open loop gain of the system substantially constant.

The linear characteristic of the valve is obtained by the use only of circular orifices which can be formed by known methods accurately and reproducibly.

What is claimed is:

1. A hydraulic spool valve comprising a body, a plurality of sets of metering ports for hydraulic fluid in the walls of said valve body, a spool within said valve body, said spool having raised lands for cooperating with said ports, in which each set of metering ports for the hydraulic fluid comprises at least one port of circular section and of a diameter slightly exceeding the width of the spool lands, and at least one port of circular section and of a diameter less than that of the spool lands.

2. A hydraulic spool valve according to claim 1, in which each set of ports comprises at least two ports of each size symmetrically disposed about the spool axis so as to avoid unbalanced radial forces on the spool.

3. A hydraulic spool valve according to claim 2 in which the said ports are of two sizes.

References Cited UNITED STATES PATENTS 2,696,196 12/1954 Adams et al. 137625.68 X 2,798,461 7/1957 Gold et al. 137625.69 2,872,943 2/1959 Easter 137625.68

M. CARY NELSON, Primary Examiner.

HENRY T. KLINKSIEK, Examiner. 

1. A HYDRAULIC SPOOL VALVE COMPRISING A BODY, A PLURALITY OF SETS OF METERING PORTS FOR HYDRAULIC FLUID IN THE WALLS OF SAID VALVE BODY, A SPOOL WITHIN SAID VALVE BODY, SAID SPOOL HAVING RAISED LANDS FOR COOPERATING WITH SAID PORTS, IN WHICH EACH SET OF METERING PORTS FOR THE HYDRAULIC FLUID COMPRISES AT LEAST ONE PORT OF CIRCULAR 